Simulated moving beds have lately been the method of choice for separating highly demanding and expensive products such as pharmaceuticals, biochemicals and fragrances. The technology has moved from its roots in the petrochemical industry to sugar separations, and is now making successful inroads into the drug industry. Guest (1997); Juza (1999); Juza (2000). This technology has received particular attention for separating enantiomers with minute differences in adsorbent selectivity. Miller, et al. (1999).
Chronologically, the first commercial simulated moving bed is credited to Broughton and Gerhold of Universal Oil Products (UOP) (Des Plaines, Ill.), whose 1961 U.S. Pat. No. 2,985,589 describes the idea of moving ports on fixed beds and an accompanying rotary valve to distribute stream flows among the fixed beds. The Simulated Moving Bed, referred to herein as “SMB,” was presented as a distillation type column with downcomers and partitions with lines leading to the rotary valve. The simulated moving bed contained a check valve to control the direction of flow and a variable speed recycle pump to move process fluid through the system. UOP has since obtained various patents that introduce a number of slightly different rotary valve designs. See, e.g., Carson et al., U.S. Pat. No. 3,040,777, Gerhold et al., U.S. Pat. No. 3,192,954, and Liebman et al., U.S. Pat. No. 3,422,848.
In 1972, de Rosset and Neuzil, both of UOP, obtained U.S. Pat. No. 3,706,812 which became the basis of current practicing lab and pilot scale SMB systems. The downcomer sections disclosed in Broughton's earlier patent were replaced with columns linked together through tees connected to rotary valves, as shown in FIG. 1. The disclosed system incorporated a check valve between each column and its tee to maintain correct directional flow. The '812 patent also disclosed the use of solenoid valves to move the ports through the columns.
While the SMB design disclosed in the '812 patent is generally considered a much improved system over Broughton's earlier design, is suffered from a number of significant drawbacks relating to cross-contamination. In particular, because the unused transit lines between the tees and the rotary valve admix into the inline flow through the tee, at a later switching time the desorbent is flushed into and contaminates the raffinate, and feed is flushed into and contaminates the extract. Similarly, the system relies upon a variable-speed pump that often disrupts separation profiles with its non-instantaneous response time and inherent mixing nature. Moreover, this system cannot be expanded or modified to fit different configurations; it cannot perform online decoupled regeneration; it does not accommodate variable zone lengths; and it does not accommodate open loop systems.
The valves employed in an SMB are critical to the expandability and flexibility of a simulated moving bed under design. SMB systems can generally be characterized by the types of valves employed in the SMB, and how the valves are arranged. The two types of valves used to switch the ports in an SMB are multi-position rotary valves and on-off valves. On-off valves include solenoid, gate, ball, plug, diaphragm (weir), butterfly and other types.
Rotary valve SMBs can generally be characterized either as centralized or distributed rotary valve systems. Centralized rotary valve systems usually rely upon a single specially designed rotary valve to distribute streams among the various columns and to implement column switching. The two rotary valves shown in FIGS. 1 and 2 are examples of specially designed rotary valves for centralized systems. The two valves are marketed by Universal Oil Product (UOP) and Advanced Separation Technology (AST). Centralized rotary valve systems suffer from the disadvantage that they only work in systems that are designed for synchronous switching. In addition, while the designs minimize cross-contamination, valves designed for centralized rotary valve systems must be specially built to work with a particular zone/column configuration and thus lack flexibility for use in varied applications.
Distributed rotary valve systems rely upon multiple rotary valves interposed between columns. These valves, such as the SD valve discussed in greater detail below, are usually generic and adaptable to many SMB designs. Rotary valves contain a rotating piece and a static piece, respectively called the rotor and the stator. The rotor rotates on a single axis and aligns the various ports on the two pieces. The most common type of rotary-valve employs a single inlet with multiple outlets. Alternatively, the ports can be interchanged to obtain a single outlet with multiple inlets. The valve acts to select a single stream from a number of dead ended streams and directs it to the valve outlet, or vice-versa. This flowpath, shown in FIG. 3a, is called the SD for Select-Dead-end. FIG. 3 contains a series of figures obtained from the internet web-site of Valco Instruments Co. Inc. (Houston, Tex.).
A number of other flowpaths are available based on Valco rotary valves. The Select-Common-outlet (SC) flowpath shown in FIG. 3b allows the non-selected streams to share a common outlet instead of being dead ended as in an SD valve. In the Select-Flow-through (SF) flowpath, shown in FIG. 3c, the non-selected streams are allowed to flow out through individual outlets instead of a common outlet. In the Select-Trapping (ST) flowpath shown in FIG. 3d, there exists a single outlet and a single inlet. The ST flowpath acts to interrupt the flowpath of a stream. Flow from the inlet goes through one of a selected pair of ports and is returned into the valve via an external loop into the selected port's mate before finally leaving the valve through the outlet. As shown in FIG. 3d, the non-selected streams can be trapped in external loops while the selected loop allows flow from the inlet to the outlet. The flowpath in a Select-Trapping/Flow-through (STF) valve, as shown in FIG. 3e, is a combination of the ST and SF flowpaths. The STF valve is similar to the SF valve except that the non-selected streams are allowed to flow out through their own ports.
Advanced Separation Technologies (AST) (Whippany, N.J.) has a number of SMB rotary valve systems that essentially employ separate pieces of valves at each end of the columns. See, e.g., Berry et al., U.S. Pat. Nos. 4,522,726 and 4,808,317. These systems evolved into the commercial CSEP systems which use the ISEP valve. The ISEP design is described by Rossiter and Riley in U.S. Pat. No. 5,676,826 (1997). As shown in FIG. 2, the ISEP design employs four constant speed pumps with two inlet pumps (feed and desorbent), an outlet (raffinate) pump and one recycle pump (Zone II).
The lower portion of the ISEP valve rotates together with the columns to achieve port switching, as shown in FIG. 2. This design avoids admixing in the tees that occurred in UOP's design, and is flexible in terms of port and zone configurations. Moreover, the design has high purity and low dead volume and is relatively simple to control. However, the system suffers from a number of drawbacks, including its high cost and the need to rotate the columns in operation. Additionally, configurations supported by the ISEP valve are limited, and because the ISEP valve employs synchronous switching it cannot be used for variable zone length and online decoupled regeneration operation. Other rotary valve systems are described by Matonte in U.S. Pat. No. 5,069,883, and Morita and Ohno in U.S. Pat. No. 5,478,475.
Multiple rotary valve systems use generic rotary valves such as the SD type (FIG. 3a) that are widely available and generally less expensive than their proprietary counterparts, though they typically have higher dead volume and more complex controls. There are two basic systems using multiple rotary valves, the one SD rotary valve per stream system (1SD1S) and the one SD rotary valve per column system (1SD1C).
The 1SD1S system in its simplest form consists of a single SD valve dedicated to each stream, as shown in FIG. 4. Priegnitz disclosed the system in U.S. Pat. No. 5,470,464. The 1SD1S design is popular for its low cost, simplicity and wide availability of parts. The SD valve is available commercially and because Valco's SD valves have up to 26 ports, which correspond to 26 columns, provides considerable flexibility. Nevertheless, while the design is efficient, it suffers from several significant drawbacks. In particular, the admixing of the stagnant lines at every manifold causes significant contamination of the inlet and outlet streams. Moreover, the design employs a variable speed pump for recycling the process stream.
A variant of the 1SD1S interrupts the raffinate and extract streams with a second SD rotary valve and incorporates additional on-off valves between the columns to create a Two SD Rotary Valve per Stream (2SD1S) system. This modification, as shown in FIG. 5, eliminates the need for a variable speed recycle pump, and instead only relies on constant speed pumps. This system is described by Negawa and Shoji in U.S. Pat. No. 5,456,825, and by Ikeda et al. in U.S. Pat. No. 5,770,088.
Priegnitz describes in U.S. Pat. No. 5,565,104 another variant of the basic 1SD1S where an STF (FIG. 3e) valve is added, as shown in FIG. 6. The STF valve allows a constant speed recycle pump to be used as in the 2SD1S system without the additional on-off valves. Storti, et al. (1992) used a slight variant of this system to withdraw an additional stream.
In the one SD Rotary Valve per Column design (1SD1C) a single SD valve is dedicated to each column. The SD valve selects the stream for its column, as seen in FIG. 7. U.S. Filter reportedly uses this design in their ADSEP SMB system, and Wu, et al (1998) reports having successfully used an ADSEP system in an amino acid separation. This system has several advantages over the 1SD1S system including: the capacity to perform variable step time operations; the ability to employ multiple desorbents within a zone; the ability to add additional columns to the system; and higher purity products due to lower volumes of admixing. The 1SD1C design suffers, however, from the fact that it requires a variable speed recycle pump and is limited to closed-loop binary separations.
A Four Two-way Valves per Column system (4-2W1C), as shown in FIG. 8, can be thought of as a replacement of each rotary valve on the 1SD1C design with a set of four two-way on-off valves. The system was first detailed by de Rosset and Neuzil in U.S. Pat. No. 3,706,812. It is often referred to as the “Sorbex” design in the literature, even though Sorbex is the trademark used by UOP for all their SMB technologies including the single rotary valve design. Novasep reportedly uses this system in their units. United States patents that disclose schemes containing “Sorbex” diagrams include Odawara et al., (U.S. Pat. No. 4,157,267), Yoritomi et al., (U.S. Pat. No. 4,379,051) and Schoenrock et al., (U.S. Pat. No. 4,412,866). “Sorbex” based SMB systems have also been mentioned in the literature by Beste, et al. (2000); Cavoy, et al. (1997) pp. 49–57; Ching, et al., (1993) pp. 1343–1351; Juza, M., (1999); Kawase, et al. (1996); Nagamatsu, et al. (1999); Navarro, et al. (1997); Pais, et al. (1997); and Pais, et al., (1998).
A variable speed recycle pump is required in the basic 4-2W1C system. Check valves, not shown on FIG. 8, are sometimes used to maintain correct directional flow. The 4-2W1C system is inherently flexible in terms of column number and zone configuration and can easily be modified to handle multi-solvent and multicomponent systems (Tanimura and Tamura (U.S. Pat. No. 5,556,546)). On the other hand, on-off valves inherently have cross-contamination and the large number of valves employed in a on-off-based system requires complex controls. The cross-contamination is more acutely felt with smaller scale systems.
Another on-off valve system, the Six Two-Way Valves per Column (6-2W1C) system, as shown in FIG. 9, removes the need for a variable speed pump. The constant speed recycle pump is placed in zone IV to minimize contamination. In a “Two Three-way Valves per Column” design (2-3W1C), the two pairs of two-way on-off valves in a 4-2W1C design are replaced by two three-way on-off valves to create the 2-3W1C system shown in FIG. 10. The 2-3W1C system and variants thereof can be found in United States patents from Golem (U.S. Pat. No. 4,434,051), Moran (U.S. Pat. Nos. 5,635,072 and 5,705,061) and Green (U.S. Pat. No. 6,004,518), all of UOP. The inventors claim high purity in their system. However, the basic requirement of a transit line to the inline flow from the on-off valve remains, thus requiring a specialized flushing procedure which reduces yield, or a redesign of the on-off valve to merge with the tee, which defeats the purpose of using the lower cost generic valves.
Regeneration of Column Packing Material
Practical industrial considerations in the operation of simulated moving beds typically require column packing material to be regenerated periodically. In theory, regeneration can be accomplished using a stronger solvent, a temperature swing, a pH swing, or a pressure swing for supercritical and gas chromatography operations. Simple SMB gradient configurations have been published by Abel, et al., (2002); for solvent gradient SMB, Migliorini, et al., (2001), for temperature gradient SMB, and Mazzoti, et al., (1997), for a pressure gradient SMB. In addition, Antis, et al., (2001), recently reviewed gradient SMB in the literature.
Unfortunately, most SMB designs today are unable to handle periodic regeneration effectively. One way to integrate periodic regeneration into the SMB operation is to manually remove the column to be regenerated, and to replace the column with a fresh column. The removed column is attached to a separate system (offline) to be regenerated. Because the separate system is decoupled from the simulated moving bed, the length of time required to regenerate the column is not a significant factor. However, this method requires the SMB system to pause while in operation, which results in unwanted spreading of the bands, and the labor-intensive operation is unattractive at best. See Xie et al., (2002).
Another way to accomplish column regeneration has been to use multiple columns in an additional regeneration zone to the conventional four zones, which allows sufficient time for the columns to be regenerated. The regeneration zone step time is then coupled to that of the regular SMB operation step time. Because the length of time required to regenerate a column typically greatly exceeds the step time observed by the simulated moving bed, these systems typically require a large number of online columns in the regeneration zone. The large number of columns reduces the average throughput per bed volume of the system and can rarely offset the benefit of coupling the regeneration step to the SMB separation steps.
Variable Zone Length/Step Time Operations
Recently, U.S. Pat. No. 6,136,198 reported a new system, known as the Varicol process, which is based on a non-synchronous shift of inlet and outlet ports, such that the zone length and/or step time of the SMB varies during operation. They reported a mathematical model which predicts experimental results quite well for the chiral separation of the 1,2,3,4-tetrahydro-1-napthol racemate for SMB and Varicol processes, and asserted that the performance of the Varicol process could exceed the performance of traditional SMB systems due to the flexibility offered by decoupling and varying the column switching times. The Varicol system is reported to offer a new dimension to SMB processing options. However, its potential use is severely hindered by the need for valve systems that can accommodate such a process.
It is also possible to maximize yield and purity, and to minimize solvent consumption, by varying the inputs, outputs and zone flowrates according to a prescribed method, basically by using variable speed pumps. Such a method is disclosed in U.S. Pat. No. 5,102,553.
Summary and Comparison of Existing Systems
FIG. 11 divides existing SMB systems by valve types and arrangements. Table 1 compares the existing systems.
TABLE 1Comparison of Existing SMB Systems.UOP/SingleRotaryAST/Daicel/UOPCompanyValveKnauerUOPPurdueUS FilterNovasepSystemUOP1SEP1SDIS2SDIS1SDIC4-2WIC/2-3WICPumps*4C + 1V3C/4C4C + 1V3C/4C4C + 1V4C + 1V/3C/5CPressureTrade Secret<1400 psi<500 psi<500 psi<300 psi<5000 psiColumnsStationaryRotateStationaryStationaryStationaryStationaryColumnNoNoNoNoYesYesExpansionMultipleTradeGoodLimitedLimitedLimitedExcellentZonesSecretFlexibilityNumber ofBinaryMultipleBinaryMultipleBinaryMultipleSolutesSolutesSolutesSolutesPossiblePossiblePossibleAdmixingTrade SecretNoneSubstantialSubstantialSomeSubstantialCross MixingTrade SecretNoneNoneSomeSomeSubstantialOpen LoopTrade SecretYesNoYesNoYesZone BypassNoYesNoYesNoPotentialParallel SMBNoYesNoPotentialNoPotentialVariable StepNoNoPotentialPotentialPotentialYesTimesMultiple SMBNoNoNoNoNoNoSchemesMultipleNoNoNoNoPotentialPotentialDesorbentsWithin 1ZoneOnlineNoNoNoNoNoNoDecoupledRegenerationValving CostVery HighVery HighLowLowMediumLowNumber of11Depends on2 × # ofDepends on(2 or 4) × #Actuators(Simple)(Simple)# ofstreams + ## ofof columns-streams-of columnscolumns-usuallyusually 4usually 14–18usually16–48(Simple)(Complex)8–12(Very(Complex)Complex)Unsteady-NoNoNoNoNo4-2WIC/State2-3WICCapacitypotential*C refers to Constant Speed Pump and V to Variable Speed Pump